MAX3267 MAXIM [Maxim Integrated Products], MAX3267 Datasheet - Page 6

• Manufacturer Part Number: MAX3267
• Description: 1.25Gbps/2.5Gbps, 3V to 5.5V, Low-Noise Transimpedance Preamplifiers for LANs
• Manufacturer: MAXIM [Maxim Integrated Products]

The MAX3266 includes a 1-pole lowpass filter which
limits the circuit bandwidth and improves noise perform-
ance.
The DC cancellation circuit uses low-frequency feed-
back to remove the DC component of the input signal
(Figure 4). This feature centers the input signal within
the transimpedance amplifier’s linear range, thereby
reducing pulse-width distortion on large input signals.
The DC cancellation circuit is internally compensated
and therefore does not require external capacitors. This
circuit minimizes pulse-width distortion for data
sequences that exhibit a 50% duty cycle. A duty cycle
significantly different from 50% will cause the MAX3266
to generate pulse-width distortion.
DC cancellation current is drawn from the input and
creates noise. For low-level signals with little or no DC
component, this is not a problem. Amplifier noise will
increase for signals with significant DC component (see
Typical Operating Characteristics ).
Many of the MAX3266 specifications relate to the input
signal amplitude. When working with fiber optic
receivers, the input is usually expressed in terms of aver-
age optical power and extinction ratio. Figure 5 shows
1.25Gbps/2.5Gbps, 3V to 5.5V, Low-Noise
Transimpedance Preamplifiers for LANs
Figure 3. MAX3266 Limited Output
6
AMPLITUDE
_______________________________________________________________________________________
Applications Information
Optical Power Relations
DC Cancellation Circuit
TIME
OUTPUT (SMALL SIGNALS)
OUTPUT (LARGE SIGNALS)
Output Filter
relations that are helpful for converting optical power to
input signal when designing with the MAX3266.
Optical power relations are shown in Table 1; the defini-
tions are true if the average duty cycle of the input data
is 50%.
The input-referred RMS noise current (I
MAX3266 generally determines the receiver sensitivity.
To obtain a system bit error rate (BER) of 1E-12, the
signal-to-noise ratio must always exceed 14.1. The
input sensitivity, expressed in average power, can be
estimated as:
Where
The overload is the largest input that the MAX3266
accepts while meeting specifications. The optical over-
load can be estimated in terms of average power with
the following equation:
Figure 4 . DC Cancellation Effect On Input
Sensitivity
AMPLITUDE
Overload
is the photodiode responsivity in A/W.
10
Optical Sensitivity Calculation
INPUT (AFTER DC CANCELLATION)
log
10
14 1
log
INPUT FROM PHOTODIODE
.
Input Optical Overload
2
I
1
N
r
mA
2
e
r
e
1000
1
1
1000
dBm
TIME
N
) of the
dBm